10 ELEMENTS OF GENERAL SCIENCE 



open end in a cup of mercury. When the finger is removed, 

 the mercury in the tube will not continue to fill the whole 

 tube but will fall so that the top of the mercury column is 

 about thirty inches above the mercury in the 

 dish (fig. 9). If this column of mercury is 

 observed from time to time for several days, 

 it will be found to rise or fall slowly. The 

 change in height of the column is due to 

 the change in pressure of the air. 



10. Graduation of the barometer. In order 

 to be an efficient instrument a barometer 

 must be graduated ; that is, a scale of some 

 kind must be attached to it in such a way 

 that the movements of the pointer or of the 

 top of the mercury column may be easily 

 read and recorded (fig. 10). This scale might 

 be marked in pounds pressure per square 

 inch, and this would seem a very natural 

 thing to do, but it is not the common method. 

 Mercury barometers were the first used, and 

 usually we express air pressure by stating 

 the height of the column in a mercury barom- 

 eter. These barometers are therefore gradu- 

 ated in inches or centimeters, and a scale of 

 this sort will be found upon any practical 

 instrument for measuring air pressure. An 

 aneroid barometer is commonly given a scale 

 FIG. 9. A simple so graduated that the numbers correspond 

 barometer with the readings of the mercury barometer, 

 The weight of the and therefore the readings of the two are 



air holds the met- . -, , . -. 

 cury in the hoi- identical. 



low glass tube. Since the air pressure decreases with increase 



Any change in the .. , . , 



weight of the air o elevation, barometers may also be graduated 

 changes the height to read in terms of the elevation above sea 



of the column of 



mercury level. Aneroids are frequently so graduated. 



